Bleaching system including a single,triple-deck table washer

ABSTRACT

APPARATUS USED IN THE BLEACHING OF PULP. PULP IS CONVEYED FROM A CHLORINATION TOWER TO A FIRST DECK OF A THREE DECK TABLE WASHER. THE WASHED PULP IS THEN CONVEYED TO A CAUSTIC TREATMENT TOWARD AND AFTER TREATMENT THEREIN CONVEYED TO A SECOND DECK OF THE THREE DECK TABLE WASHER. AFTER THIS SECOND WASH, THE PULP IS TREATED IN A HYPOCHLORITE TOWER AND THEREAFTER WASHED ON A THIRD DECK OF THE TABLE WASHER. EACH DECK OF THE TABLE WASHER HAS A CIRCULAR, ROTATABLE, HORIZONTAL SCREEN THEREON AND A VACUUM ACTS TO DRAW THE EFFLUENT WASH WATER THROUGH THE SCREEN. THE TABLE WASHER CAN WASH PULP AT A CONSISTENCY BETWEEN 1 1/2 AND 5 PERCENT.

United States Patent Inventor Alfred Max Hurter Beaconsfield, Quebec,Canada Appl. No. 766,045 Filed Oct. 9, 1968 Patented June 28, 1971Priority Oct. 16, 1967 Great Britain 47130/57 Assignee to Stadler HurterInternational Ltd., Montreal, Quebec, Canada BLEACIIING SYSTEM INCLUDINGA SINGLE, TRIPLE-DECK TABLE WASHER 68/184; 210/(Horizontal FilterDigest), 325; 8/156; 68/181; 162/17, 19, 60, 88, 8 9, 242, 41, 43

'u'HIlLSkCH Ell P lLF UNHLE L'AI'MBED PULP 5 FILIRAI'E e TANK CHI/1R! NE1ST HRUSTILI TONER Primary Examiner-S. Leon Bashore Assistant ExaminerA.DAndrea, Jr.

Attorney-Alan Swabey ABSTRACT: Apparatus used in the bleaching of pulp.Pulp is conveyed from a chlorination tower to a first deck of a threedeck table washer. The washed pulp is then conveyed to a caustictreatment tower and after treatment therein conveyed to a second deck ofthe three deck table washer. After this second wash, the pulp is treatedin a hypochlorite tower and thereafter washed on a third deck of thetable washer. Each deck of the table washer has a circular, rotatable,horizontal screen thereon and a vacuum acts to draw the effluent washwater through the screen. The table washer can wash pulp at aconsistency between 1 6 and 5 percent.

STORAGE CHE? SECOND STA/3E HYPOCH'LORITE TONER HLEACHED PULP CONS ISTEXCY RHSULATUR BLEACIIING SYSTEM INCLUDING A SINGLE, TRIPLE- DECKTABLE WASHER This invention relates to a compact and simplified pulpbleaching system and a method of bleaching pulp.

The bleaching system is designed to make it economically feasible foruse with a pulp mill having a production as small as 40 to 50 tons ofpulp a day or as large as 300 tons of pulp per day by utilizing aminimum amount of simple equipment par' ticularly in the amount ofwasher equipment required. The system is particularly designed to use atleast one table washer for washing the pulp between treatment stages inthe bleaching system. A single table washer can be made to serve up tosix treatment stages in the'bleaching system for production of 100 tonsof pulp a day or less. The system, in using a table washer, makes veryeconomical and efficient use of the wash water use in the bleachingsystem. The pulp can be washed at relatively high consistency rangingbetween 1% and 5 percent and, therefore, very little fresh water isrequired in the system. Because of the small amount of fresh waterrequired, most of the filtrate from the washer can be led directly to asewer. Since only a small part of the filtrate is reused in the system,only one filtrate tank and pump is required.

The bleaching system is further designed to use a minimum number ofstock pumps. Only two pumps are used in the bleaching system. If thepumps at the unbleached pulp supply chest and the bleached pulp storagechest are included, only four pumps are required in the system which isless than that non'nally used in a bleaching system.

The invention is particularly directed toward a method of bleaching pulpincluding the steps of passing the pulp through a plurality of bleachingstages in succession to bleach the pulp. The pulp is passed between atleast two successive bleaching stages to a washer at a consistencyranging between 1% and 5 percent.

The invention is also directed toward a system for bleaching pulpincluding a plurality of treatment towers for bleaching the pulp andmeans for feeding the pulp to be bleached through the towers insuccession. The system includes a washer capable of washing pulp havinga relatively high consistency ranging between 1% and 5 percent andincludes means for feeding the pulp during passage between at least twosuccessive towers to and from the washer. The system is particularlydesigned to use a table washer.

A preferred embodiment of the bleaching system will now be described indetail having reference to the FIGURE which illustrates a flow sheet ofthe bleaching system.

The bleaching system illustrated uses a three deck table washer in aplant especially adapted for bleaching pulp obtained from cottonlinters. However, the system could serve equally well for the bleachingof bagasse, straw, reeds, jute, hemp, and even some hardwood kraft pulpsor softwood sulfite pulps if these pulps do not require a high level ofbrightness.

As shown in the drawing, the unbleached pulp is fed into the bleachingsystem through line 1 to an unbleached pulp chest 3. From the chest thepulp is pumped by stock pump 5 through a consistency regulator 7. Theconsistency regulator determines the arnount of water added through line9 to the pulp to obtain a consistency of approximately 3 A percentrequired for the next operation. From the consistency regulator the pulppasses through a flow control valve 11 to a chlorine mixer 13 wherechlorine, added through line 15, is intensively mixed with the pulp. Themixer preferably has a retention time of one minute to obtain thoroughmixing. From the chlorine mixer the pulp-chlorine mixture is passed tothe bottom of a chlorination tower 17. The tower is a simple cylindricaltank having a dished top and a dished or flat bottom and no internalfittings or appurtenances. At the top and bottom of the tower are twopropeller agitators 19 for agitating the chlorinated pulp. Theseagitators preferably are fixed blade, bare shaft, cantilevered, radialunits with outboard bearings. From the top of the tower the chlorinatedpulp flows through line 21 and, through gravity, to a hydraulic mixer23, located below the top of the chlorination tower. The pulp id dilutedto a consistency of approximately 2% percent in the hydraulic mixer 23by adding fresh water from line 25. At this consistency the pulp flowson to the first stage or deck 27 of a three-deck table washer 29. Thepulp is washed by fresh water from line 3], fed from supply line 32,leading to the first stage 27. The filtrate from this washer flowsthrough line 33 to the sewer.

The washed pulp is removed from the first stage 27 of the table washer29 by a feed conveyor 35 at a consistency of [2 percent and fed to asteam and chemical mixer 37 where caustic (NaOl-l) and steam are addedto the pulp from lines 39 and 41 respectively. FRom the mixer the pulpfalls into the caustic tower or tank 43. This tower again is a simplecylindrical tank with dished top and dished or fiat bottom and withoutspecial internal fittings or appurtenances. At the bottom of this tower,there is another radially mounted propeller agitator 45 similar to theones in the chlorination tower 17. A single low pressure dilution line47 is provided within the caustic tower. The pulp within the tower isdiluted with filtrate through line 47 pumped from a filtrate tank 49 bypump 51. The pulp passes from the bottom of the caustic tower, isdiluted to a consistency of about 3 percent by filtrate from filtratetank 49 through line 53, and pumped by stock pump 55 through line 57 tothe second stage 59 of table washer 29. Here it is washed by fresh watersupplied through line 61 from supply line 32. The filtrate from thesecond state 59 of the table washer is passed to the sewer through line63.

From the second stage of the table washer, the pulp is removed at l2percent consistency by feed conveyor 65 and feed to a steam andhypochlorite mixer arrangement 67. Because of the rapid and irregularrate of reaction that could occur if the hypochlorite and steam weremixed simultaneously, one mixer 69 for the steam and one mixer 71 forthe hypochlorite, are used in series. STeam is fed to the steam mixer 69through line 73 and hypochlorite is fed to mixer 71 through line 75.

From the mixers 69, 71 the pulp is fed to a first stage hypochloritetower 77 which is identical, except for being taller, to the caustictower 43. AT the bottom of the hypochlorite tower the pulp is diluted toa consistency of approximately 6 percent with the filtrate from filtratetank 49 through line 79. A single agitator 81 is located at the bottomof the hypochlorite tower and is of the same design as those previouslydescribed.

From the bottom of the first stage hypochlorite tower the pulp is pumpedby pump 83 to an upflow second stage hypochlorite tower 85. if needed, afurther adjustment to the pH or bleach of the pulp can be made by addingchemicals to it at the suction side of pump 83 through one or more lines87. The second stage tower 85 is provided with a conical bottom. At thetop of the upfiow tower, fresh water is added through line 87 to dilutethe pulp to a consistency of 3 percent. An agitator 89 is located at thetop of the upflow tower. From the top of the second stage tower the pulpis fed to the third stage 91 of the table washer 29 through line 93where it is washed with fresh water from line 95 supplied through line32. The filtrate from the third stage 91 flows through line 97 to thefiltrate tank 49. The filtrate from the filtrate tank 49 is used todilute the pulp to a required consistency at various points in thesystem through lines 47, 53 and 79 as previously described.

The washed pulp is removed from the third stage by feed conveyor 99 ontoa table conveyor 101 and is fed to a high density stock storage chest103 having an agitator 105 at the bottom. The bleached pulp can bewithdrawn from the storage chest by pump 107 for further use in the pulpmill with its consistency regulated by consistency regulator [09.

The three-deck table washer 29 shown in the bleach system is a standardtype of pulp washer well known in the art. Each deck or stage of thewasher generally comprises a circular, rotatable, horizontal screen ontowhich the pulp is fed. The pulp is carried around by the screen whilewash water is directed onto the pulp, passing through it and the screenaided by a vacuum condition under the screen. The table washer 29 usedis of the type manufactured, for example, by MultiFiber Process Ltd., aCanadian company in Montreal.

The use of table washers permits the bleaching system to use less waterin the washing system than the standard bleaching systems which usecylinder or drum washers. The table washers permit the pulp to be fedonto the washers at a relatively higher consistency, ranging from W;percent to 5 percent, as compared to a consistency of approximatelyonehalf percent required by cylinder washers. This permits a reductionin the amount of water to be handled by the system and thus a reductionin the equipment required. A further advantage is that fresh water canbe used for diluting the pulp coming from the chlorination tower 17rather then using the filtrate from the third and second stages incounterflow direction to the first stage as is generally done in usingtable washers in other applications The filtrate from the first andsecond stages can instead be directly led to the sewer. Only thatfiltrate from the third stage of the table washing is collected fordiluting the pulp in the system. Therefore, only one filtrate tank andone filtrate pump is required in the system.

For pulp mills producing only 50 tons of pulp a day, one large diametersingle deck table washer can be used in the bleaching system havingthree feed and three takeoff points equally spaced about the table. Forproduction of l00 tons of pulp a day, a two-deck table washer normallyis required, and for production of 150 to 300 tons of pulp a day, threedecks, as shown in the preferred embodiment, would be used.

in addition to the simplified design and low cost, the bleaching systemdescribed, using a table washer, is ideally suited for processingbagasse pulp. Many agricultural residues such as bagasse do not form agood filter cake on a standard drum or cylinder washer generally used inbleaching systems. in fact, pulps produced form agricultural residues,such as cotton linters, hemp and jute, cannot be picked up by a drumwasher. In the case of a table washer, the stock merely flows onto thehorizontal table, and there is no question of picking up and forming afilter cake as is required by a standard drum washer. The use of tablewashers overcomes the problems in bleaching systems of washingagricultural residues which cannot be effectively washed on drumwashers.

While the system shown in the preferred embodiment uses four treatmenttowers with a three deck or stage table washer, it will be obvious thatmore or less towers may be used to bleach the pulp along with more orless decks or stages in the table washer.

The system described has five identical pulp agitators in the treatmenttowers. ln a smaller plant, the agitator 19 at the bottom of thechlorination tower can be eliminated and the tower can be fitted insteadwith a conical bottom.

All the treatment towers 17, 43, 77 and 85 used in the system are of thesame to reduce fabrication costs and preferably made of rubber-linedsteel.

lclaim:

1. Apparatus for use in making pulp including at least two bleachingmeans, a single table washer having at least two separate washingsections, first means for directing pulp from one of said bleachingmeans onto one section of said table washer where the pulp is washed,second means for feeding the washed pulp from the one section of thewasher to the other of said bleaching means, third means'for directingthe pulp from the other bleaching means after bleaching onto the othersection of said table washer where the pulp is washed, and fourth meansfor feeding the washed pulp from the other section after washing.

2. Apparatus as claimed in claim 1, wherein said first and third meansinclude means for diluting the pulp to a consistency ranging between IV;and 5 percent prior to its entry onto the sections of the table washer.

3. Apparatus as claimed in claim I, wherein the single table washercomprises at least two decks, each deck forming one section of the tablewasher.

4. Apparatus as claimed in claim I, including means for collectinfiltrate from only one section of the table washer.

5. pparatus as claimed lll claim 1, wherein each bleachlng meansincludes at least one bleaching tower.

6. Apparatus as claimed in claim 1, wherein at least one of said firstand third means includes a pump for pumping the pulp to the tablewasher.

7. Apparatus for use in making pulp including means for treating thepulp with chlorine, means for treating the pulp with caustic, and meansfor treating the pulp with hypochlorinate, a single table washer havingthree separate washing sections, first means for directing pulp form thechlorination means onto a first section of said table washer where thepulp is washed, second means for feeding the washed pulp from the firstsection of the table washer to the means for treating the pulp withcaustic, third means for directing the pulp from the means for treatingthe pulp with caustic to a second section of said table washer where thepulp is washed, fourth means for feeding the washed pulp from the secondsection to the means for treating the pulp with hypochlorinate, fifthmeans for directing the pulp from the means for treating the pulp withhypochlorinate to a third section of the table washer where the pulp iswashed, and sixth means for feeding the washed pulp from the thirdsection of the table washer.

8. Apparatus as claimed in claim 7, wherein at least one of said first,third and fifth means includes means for diluting the pulp to aconsistency ranging between 1% and 5 percent prior to its entry onto thesections of the table washer.

9. Apparatus as claimed in claim 7, including means for collecting thefiltrate from only one of the sections of the table washer.

10. Apparatus as claimed in claim 7, wherein the first and fifth meansinclude pumps for pumping the pulp to the respective sections of thetable washer.

11. Apparatus as claimed in claim 7, wherein the table washer comprisesthree decks, each deck forming one section of the table washer.

